The present invention relates to a heating method and heating apparatus for carrying out a residual-stress-relief treatment by induction heating (Induction Heating Stress Improvement) of pipe line systems in industrial plants and especially of welded joints between main and branch pipes and portions adjacent to the welded joints in nuclear power plants under construction or in operation.
Recently, the residual-stress-relief treatment (Induction Heating Stress Improvement) has been widely carried out to eliminate tensile stresses remaining in the inner surfaces of pipes due to heating effects at welded joints in pipe line systems or to change such residual stress into compression stress in nuclear power plants under construction or in operation.
In the pipe line systems as described above, an extremely large heating energy is transferred to pipes when they are joined by welding so that residual stress grows. As a result, the pipe lines tend to be reduced in strength and to be highly corrosive. For instance, when the operation of a nuclear power plant is started without making any treatment of welded joints in a pipe line system (especially a pipe line system using Type 304 austenitic stainless steel), high-temperature and high-pressure liquid flows through the pipe line system. The liquid is highly corrosive and the repeated thermal stress is caused in the pipe line system. Therefore, the tensile stress or residual stress grown in the portion adjacent to the welded joint due to the welding synergetically effect with the above-described adverse environments so that the fatigue strength is decreased. Furthermore, it has been found that anticorrosive property is decreased by chrominum carbide precipitated in intergranular in material so that so called intergranular corrosion cracks result. Therefore the residual-stress-relief treatment is carried out in order to prevent such intergranular corrosion cracks.
The residual-stress-relief treatment is such that the tensile stress caused in the inner surface of the pipes adjacent to the welded joint is eliminated or changed into the compression stress. The treatment is carried out as follows: First the inner surface of the pipe is cooled by liquid while only a portion adjacent to the welded joint is locally heated by a suitable heating means from the exterior so that a suitable temperature difference for relieving the stress is produced between the outer and inner wall surface of the heated portion, whereby the thermal stress in excess of a yield point is caused in the heated portion. Thereafter, the heated portion is cooled to room temperature while the liquid flows through the pipe line system so that the temperature difference between the outer and inner wall surfaces is eliminated. When such residual-stress-relief treatment is carried out in a pipe line system in an actual plant, the following problems arise.
FIGS. 1, 2 and 3 show devices which are disclosed in the Japanese Pat. No. 1207318; Japanese laid open application No. 58-135593; and U.S. Pat. No. 4,505,763, respectively and which are to be effective in carrying out the residual-stress-relief treatment of the welded joint between a main pipe and a branch pipe which is inclined at a suitable angle, and the welded joint between a branch pipe and a pipe base (a pipe structure consists of main pipe and short-pipe-like branch seat which has a suitable diameter and a suitable wall thickness and is welded to the main pipe) or main pipe and branch coaming. (The term "welded joint" used in this specification includes welded joints of the types described above). However, any of these devices cannot attain satisfactory residual stress relief. First, the device A as shown in FIG. 1 is apparently very much complicated in winding fabrication. Moreover, at the welded joint which is the most important part, an electric current flow changes its direction from the circumferential direction of the main pipe to the circumferential direction of the branch pipe or vice versa so that the magnetic flux density distribution is not uniform. As a result, it is difficult to control the temperature so that it becomes difficult to obtain a uniform temperature distribution. Furthermore, the branch pipes which are subjected to the residual-stress-relief treatment are different in size and shape so that a full-size branch pipe mock-up must be prepared based upon the measured data and an inductor must be modified many times based upon the full-size branch pipe mock-up until a satfisfactory inductor is designed. That is, an inductor to be used is fabricated through the so-called mock-up tests. As a result, there arises the problem that much time and expense are needed to design and fabricate a satisfactory device A.
The device B as shown in FIG. 2 has the same problems as described above. That is, no uniform temperature distribution can be obtained. In addition, the winding arrangement at the welded joint b.sub.1 is very much complicated. As a result, it is extremely difficult to reproduce the same device so that a uniform temperature distribution is difficult to attain.
In this device B, in order to improve temperature controllability, ferromagnetic members b.sub.4 and b.sub.5 are interposed between the pipes and the coil at the points b.sub.2 and b.sub.3 at which the direction of the electric current flow changes so that a uniform temperature distribution may be obtained. In order to design and construct the device B, the mock-up tests are needed as in the case of the device A described above with reference to FIG. 1, thereby determining the sizes and positions of the ferromagnetic members b.sub.4 and b.sub.5.
In the case of the device C as shown in FIG. 3, a conductor C.sub.1 is cocentrically wound about a branch pipe to cover the same and then is so wound as to cover the main pipe. The device C is different from the devices A and B in this respect. The conductor C.sub.1 is wound around the main pipe not so as to completely cover it so that a lower portion C.sub.2 of the main pipe in opposite relationship to the branch pipe is not heated. As a result, the stress is not balanced and the stress in the welded joint change from compression stress into tensile stress so that the effects of the residual-stress-relief treatment are reduced.
In summary, the fact that the welded joint and a predetermined portion adjacent to the welded joint including a zone in which no residual stress is caused are not wholly heated, adversely affects the residual-stress-relief treatment. For instance, assume that a pipe base W as shown in FIG. 4(I) is wholly and uniformly heated by a coil K and that the inner surface of the main and branch pipes be cooled by a coolant. Then tensile stress opposite to the residual stress is caused in the outer surfaces of the main and branch pipes while the compression stress is caused in the inner surfaces of the main and branch pipes constituting the base W. Let r denote the radius to the center of the wall of the branch pipe; let t.sub.1 denote the wall thickness of the branch pipe; let R denote the radius of the main pipe; and let the thickness t.sub.1 /r&lt;&lt;1. Next let us consider that the pipe base W can be separated into the branch pipe W.sub.1 and the main pipe W.sub.2 as shown in FIG. 4(II). To the ends W.sub.3 and W.sub.4, the moment Mo given below ##EQU1## where E: Young's modulus,
.alpha.: coefficient of linear expansion, PA1 .DELTA.T: temperature difference between the outer and inner surfaces, and PA1 .upsilon.: Poisson ratio
equally acts in the directions indicated by the arrows. In practice, at the welded joint of the pipe base W, moment Mo is balanced so that the above-described stress is caused.
However, when a branch pipe is uniformly heated but a main pipe is partially heated, a bending moment acting on the branch pipe is different from that acting on the main pipe. As a result, the balance between the moment Mo is collapsed so that the welded joint on the side of the main pipe is attracted into the branch pipe and the tensile stress is caused in the inner surface of the welded joint. Thus the residual-stress-relief treatment is ineffective.
In view of the above, the present invention has for its object to provide a heating method and heating apparatus which can subtantially overcome the above and other problems encountered in the conventional residual-stress-relief process and in which temperature control as well as winding arrangement can be simplified and which can improve the effects of the residual-stress-relief treatment.